Contractual consent to allow monitoring of your email\Internet access is just that, Consent. This can be formally rescinded at any point. Your employer can take action against this (Disciuplinary etc.) but they CAN NOT continue to monitor your information.

“Fishing” for issues by looking through staff email\Internet traffic will not stand up in court as there needs to be a defined threat under investigation, to remove the possibility of entrapment.

Most companies contracts or Acceptable Use Policies define that a limited amount of personal use of company resources is allowed. With this in mind, if the company monitors your email\Internet Access they are knowingly potentially viewing personal information without direct consent. This can be viewed as a breach of privacy. This can be, and has been, legally stated as a breach of Article 8 of the Human Rights Act (the right to respect for private and family life)

None of this necessarily means you can get away with things by arguing the above points, but It does mean that Information Security \ HR have to tread very carefully whilst investigating staff mis-behaviour.